As a hydraulic brake system for a vehicle, there is known a system equipped with a pressure regulator (the so-called regulator) as disclosed in the following Patent Literature. The pressure regulator provided in the system has a function of regulating a pressure of a working fluid supplied from a high-pressure-source device, as a high-pressure source, constituted by a pump, and so on, to a pressure in accordance with a pilot pressure introduced into the pressure regulator and supplying the working fluid whose pressure is regulated. More specifically, the pressure regulator has two pistons disposed in series for activating a valve mechanism. In the pressure regulator, there are formed a first pilot chamber on one side of one of the two pistons and a second pilot chamber on another side of the one of the two pistons such that the second pilot chamber is interposed between the two pistons. A pressure of the working fluid to be supplied from the master cylinder device to the brake device provided for each wheel is introduced into the first pilot chamber as a first pilot pressure. A pressure of the working fluid which is supplied from the high-pressure-source device and whose pressure is adjusted by a pressure adjuster constituted by a pressure-increase linear valve and a pressure-decrease linear valve is introduced into the second pilot chamber as a second pilot pressure. According to the structure, the pressure regulator is configured to regulate the pressure of the working fluid supplied from the high-pressure-source device to a pressure in accordance with one of the two pilot pressures and to supply the pressure-regulated working fluid.